1. Field of the Invention
The present invention relates to a control of vehicle automatic transmissions and more particularly to control means for a torque converter lock-up mechanism.
2. Description of the Prior Art
A torque converter generally includes a pump impeller connected with an engine output shaft to be driven thereby, a turbine runner having an output member and a stator disposed between the pump impeller and the turbine runner, so that hydraulic fluid is circulated by the engine driven pump impeller through the turbine runner with the aid of the stator, which functions to deflect the hydraulic fluid from the turbine runner to a direction wherein the fluid flow does not disturb the rotation of the pump impeller when the hydraulic fluid flows into the pump impeller. With the circulation of the hydraulic fluid in this way, the turbine runner is subjected to a hydrodynamic force which drives the turbine runner. The torque converter has an automatic speed changing function wherein a torque amplification is made when the speed of the turbine runner is lower than that of the pump impeller and the output torque is gradually decreased as the speed of the turbine runner approaches that of the pump impeller. However,the torque converter is disadvantageous in that it inherently has slippage between the pump impeller and the turbine runner, which causes a decrease in transmission efficiency and poor fuel economy.
It has therefore been proposed to provide a lock-up clutch mechanism between the input and output shafts of the torque converter and have the lock-up clutch engaged when the turbine runner speed has become close to the pump impeller speed so that the turbine runner is rotated as a unit with the pump impeller. For example, Japanese Patent disclosure No. 56-138559 discloses a torque converter lock-up control system which has a memory map provided with a lock-up control line defining a lock-up zone and a lock-up release zone as a function of the engine speed and the engine load. The system has an engine speed detector and an engine load detector, and there is provided a lock-up control circuit which judges, depending on the signals from the detectors, whether or not the engine operating condition as determined by the engine speed and the engine load is in the lock-up zone.
This lock-up control system is considered as being advantageous in that torque converter lock-up is automatically controlled in a preferable manner in accordance with the engine operating condition, thereby resulting in improved fuel economy. It should however be noted that in the known torque converter lock-up system there is a possibility that the lock-up mechanism may be engaged even when the engine throttle valve is fully closed, if the engine operating condition is in the lock-up zone. This provides instability of operation because the engine output torque may fluctuate when the engine throttle valve is fully closed, so that vibration or unstable engine operation may be produced if the torque converter is locked up.
In Japanese Patent disclosure No. 56-39353, it is therefore proposed to release torque converter lock-up at or in the vicinity of the full close position of the engine throttle valve, irrespective of the relationship between the engine operating condition and the lock-up control line. However, the proposed system is not satisfactory in three respects. At first, it should be pointed out that the proposed system is still disadvantageous in respect of fuel economy. In fact, under an operating condition in which the engine throttle valve is slightly opened when the vehicle speed is relatively high, the engine is rotated at an unnecessarily high speed by being driven by the turbine runner if the torque converter is locked up, drawing an unnecessarily high amount of fuel to the engine.
Secondly, it should be pointed out that a substantial shock will be produced by the disengagement of the lock-up mechanism when the throttle valve is closed. Finally, in operation on a downslope, there will be an abrupt change in the engine braking effect when the lock-up mechanism is disengaged by closing the engine throttle valve. A similar abrupt change will also be experienced when the throttle valve is gradually opened and the lock-up mechanism is engaged.